First of all, given Inquisition's narrative, I think that all DLC needs to be explicitly post-game - we need to focus on the future of the Inquisition as an organization, what they're going to be doing now that the things that drew it together, the mage-templar conflict, the Breach, Corypheus, these things have been resolved, and now there's this very powerful organization that can shape the future of Thedas. So this needs to be something that is explicitly addressed - I don't think anyone wants to learn in an off-handed fashion 'oh, yeah, the Inquisition just disbanded, that happened' in DA4. We need to get the Inquisition's future established here and now.
I think we all expect that there will be SOMETHING forthcoming with Solas. That's way too much of a dangling plot thread to leave alone.
There's a lot that makes me think that, if it doesn't tie in to things with Solas, there will be some kind of 'untold history of Skyhold' DLC. It's really convenient that there's this forgotten fortress in the middle of the Frostbacks, and there's plenty unexplained about it. I really think that something should be done to further investigate the Inquisition's base of operations.
Some kind of Warden based DLC would be appreciated, something that allows us to see what's become of the Awakening Wardens, Nathaniel, Oghren, Velanna, and Sigrun. (Which definitely means that the later two need to get added into the Keep. *pointed glare*). Maybe they're tracking down their friend, the Hero of Ferelden, because of their research into the Calling. Patrick Weekes has already spoken about how he thinks of the HoF's story being done, but we could at least have them following their footsteps, learning if something's come together about the Calling and reversing its effects.
Also, I think that there's a place for pretty much all the previous DA2 romances and Zevran to have appearances in Inquisition's story, so their absence is pretty glaring. This NEEDS to be fixed.
I really don't want to see travel beyond the Ferelden-Orlais borders right now, mostly because there's enough to do right there that I would rather they save that up for events in DA4. But I would like to return to Denerim and other Origins locations and see more of Val Royeaux.
To conclude, I would like to see a character-based DLC that gives all the characters a fleshed out role and a wrap up. Almost all the character arcs feel unfinished to me, they need to get finished up and brought to a conclusion, since I think it's unlikely that a majority of these characters will reappear in any kind of significant fashion in future games. They need more to them than what they have.